The structure and development of the categories underlying transitive sentences in English will be examined in 80 children between 2 and 4 years of age. It is hypothesized that children use the same cognitive principles in categorizing sentences as has been demonstrated for other natural categories. That is, sentences will be organized in terms of the degree of similarity to a central, prototypical member. Both the degree of prototypicality of various sentences (differing in the type of verb they contain and the animacy of their participants) and the prototypicality of the sentence participants (differing in such factors as the degree of control over the event or the amount of perceptible change in the patient, etc.) will be examined. Children's judgments of prototypicality will be tested indirectly by training them to identify sentence participants and then testing their generalization to sentences that differ in specified ways from the training sentences. The proposed research will serve as a bridge between previous studies investigating event perception in prelinguistic infants and studies investigating semantic categories in older preschoolers, thus providing useful information about early cognitive development. It will also produce data on the normal sequencing of language comprehension which will be of use to professionals engaged in the design and implementation of infant and preschool programs, as well as those involved in the diagnosis and treatment of special populations of children, such as those with language disorders or developmental delays.